1. Field of the Invention
The present invention relates to an image mixing circuit used in such devices as video cameras.
2. Description of the Prior Art
Various image mixing circuits for obtaining an image signal with a wide dynamic range from image signals obtained at different exposure levels have been proposed. One device typical of such image mixing circuits as proposed in Japan Unexamined Patent Publication (kokai) 63-306779 (corresponding to U.S. Pat. No. 5,162,914 and No. 5,638,118 both issued to Koji Takahashi et al.) is described below with reference to FIGS. 15 and 16.
FIG. 15 is a block diagram of the conventional image mixing circuit. As shown in FIG. 15, this image mixing circuit comprises threshold value generators 101a and 101b, comparators 102a and 102b, evaluator 103, and selector 104.
The signal levels of image signals Slong and Sshort generated for images obtained with different exposure levels are compared with separate specific threshold values Th1 and Th2. Mixed image signal Smix is then generated by the selector 104 appropriately selecting image signals Slong and Sshort based on the results of this comparison.
FIG. 16 is a waveform diagram of the image signals Slong, Sshort, and Smix when an image signal containing a color modulation component is generated by a conventional image mixing circuit. The abscissae in FIG. 16 show the pixel position, and the ordinates show the amplitude of the signal wave; image brightness is zero at the origin, and increases to the right.
As shown in FIG. 16, when the image signal selection circuit of this image mixing circuit changes the output signal selection, a discontinuity in the gradation characteristic of the output signal level to the incident light level results at the point at which the output signal selection changes. This is because the synthesized image is generated using image signals from images that have been exposed at different levels.
Discontinuity in the gradation characteristic of the color signal also results when signals containing a color signal superposed on the luminance signal by a single imaging element are mixed.
The color signal hue component also changes when non-linear gradation correction, such as knee correction, is applied to the mixed luminance signal components because the ratio of the luminance signal component to the color signal component is not kept constant.
A non-linear change in the gradation of the luminance signal component also occurs when the signal with the superposed color signal component is saturated, thus resulting in a change in hue. When the amount of the per-line color modulation component also changes due to the arrangement of the color filters used with the imaging element, a difference in the luminance signal level also occurs between lines.
Furthermore, when signal levels are matched by adjusting the gain of image signals generated at different exposure levels as a means of achieving a continuous gradation characteristic, the signal-to-noise ratio in high luminance areas of the signal deteriorates according to the exposure ratio of the signals.